The reaction between A and B involves not just ions but electrically neutral
atoms. Hence for the reaction to proceed there must be another path whereby electrons
can also move through the system. This is typically an external electrical
circuit connecting A and B. If A is transported in the system, and the electrolyte
contains A+ ions, negatively charged electrons, e−, must pass through the external
circuit in equal numbers, or at an equal rate, to match the charge flux due to the
passage of A+ ions through the electrolyte to the other side.
For an electrochemical discharge reaction of the type illustrated in Fig. 1.2 the
reaction at the interface between the phase A and the electrolyte can be written as